This application relates, in general, to surveillance devices, and more particularly to electromagnetic systems for detecting intruders within an area to be protected, which system is capable of maintaining a high degree of sensitivity with high reliability against false alarms.
This application is related to, and is an improvement over, the motion detecting system described in U.S. Pat. No. 3,967,283, which is an intrusion detector that is designed to provide surveillance over large areas. The present invention retains many of the features of the earlier patent, but also includes means for restricting the response of the intrusion detector to a closely defined series of overlapping range zones which permit the system to operate with greatly improved signal-to-noise ratio, improving sensitivity, and reducing false alarm rates while still covering large areas. In addition, use of narrow beam antenna permits the system to be used as a line protector, as opposed to a large area protector, so that it can be used as a virtual fence which will detect the presence of intruders in a narrow, elongated area.
As was set forth in the aforementioned U.S. Pat. No. 3,967,283, intrusion detectors which utilize electromagnetic or acoustic signals to detect moving objects within the field defined by the transmitted signals are well known, a variety of systems having been developed over the years. However, such systems have tended to be overly complex and thus extremely expensive, and in certain cases even the more complex systems have been incapable of providing the type of protection required. Although U.S. Pat. No. 3,967,283 provided an improved system for surveying an enclosed area of a specified maximum range and included means for eliminating specific target responses to prevent interference from targets outside the area being protected as well as from dynamic clutter within the area being protected, even this system has its limitations, for in some circumstances it was found that the large area of protection provided by the patented device did not meet the requirements of certain surveillance applications. For example, if it were desired to protect an aircraft standing on a runway, the large area motion sensor could do so, but would at the same time prevent desired activity, such as repair and maintenance work, from being carried on at the same time. Thus, the system described in the aforementioned patent detected and responded to desired activity as well as undesired activity.
Another difficulty with prior systems such as that disclosed in the aforesaid U.S. Pat. No. 3,967,283 was the fact that when a large area is being covered by a surveillance system and the system responds to the existence of an intruder, there is no way to determine where the intrusion occurred. Where a very large area is being protected, this failure to provide an exact location can result in some difficulties, if not danger.